The present invention relates to an ultrafine particle and ultrafine wire forming method for forming on an insulative substrate semiconductor or metallic ultrafine particles and metallic ultrafine wire having such minute electric conductivity as is just enough to develop a quantum size effect, and to a semiconductor device using such ultrafine particles and/or ultrafine wire formed by the method.
In recent years, large-scale integrated circuit (LSI) technology which has supported the progress of electronics as a basic industrial segment has brought about striking improvements to semiconductor devices through miniaturization, the improvements including larger capacity, higher speed, and lower power consumption. On the basis of the operating principle of the prior art device, however, it is believed that when the size of the device is reduced to 0.1 .mu.m or less, the threshold limit is already reached. Therefore, research activities have been actively made for development of a new device based on a new operating principle.
Above mentioned "new device" means a device having an ultrafine structure, such as an ultrafine particle of nanometer size (hereinafter referred to as quantum dot) and/or an ultrafine wire (hereinafter referred to as quantum fine wire). With respect to quantum dots, research activities have been briskly made for application of such dots to various one-electron devices utilizing Coulomb blockade phenomenon in particular as well as for application to a variety of quantum effect devices. With respect to quantum fine wires, it is expected that application of such a wire to ultrahigh speed transistors utilizing quantum effect could be achieved.
Whilst, as a new move directed toward future development of electronics technology, attempts have been made to employ an electronic circuit and an optical communication circuit on an integrated basis. For this purpose, a photoelectric conversion device loaded on the LSI board is indispensable, and such a device, formed from an Si (silicon) series material as the main current of LSI, has been already put in practical use. Regarding the matter of light emission, since a semiconductor of Si series, IV group has an indirect transition type band gap, it has been believed in the past that the semiconductor does not emit light. Recently, however, it has been discovered that fine crystal particles of the semiconductor with a particle size of not more than 10 nm take a structural configuration of a direct transition type band and can emit light. This discovery has encouraged research activities on the utilization of such semiconductor particles.
Besides the above mentioned, there have been made a large number of research works on the quantum dot and/or quantum fine wire forming technology which are intended for application of such a semiconductor to many kinds of electrical and optical devices utilizing the principle of quantum effect. In this conjunction, a quantum dot forming method and application to one-electron transistors and light-emitting devices of quantum dots formed by the method (see Japanese Patent Application Laid-open Publication No. 8-64525) will be briefly explained.
FIG. 13F shows a section of a formed single electron transistor. A voltage is applied between a source terminal 1 and a drain terminal 2, and a current flowing between the source and the drain through crystal Si fine particles (hereinafter referred to as fine particles) 3 is on/off switched by a voltage applied to a gate terminal 4. In case where no voltage is applied to the gate terminal 4, no current flows to fine particles because of a Coulomb blockade phenomenon occurring to fine particles 3 due to quantum size effect. That is, the device is in off condition. When a voltage is applied to the gate terminal 4 to reduce the tunnel resistance between fine particles 3 to a level lower than the quantum resistance (h/4 (e * e), where h: Planck constant; e: elementary electric charge), the Coulomb blockade is broken to allow current flow. That is, the device is in on condition.
The one-electron transistor is formed in the following way. As shown in FIG. 13A, an SiO.sub.2 film 6 having a thickness of 2500 .ANG. is formed by conventional selective oxidization method on a surface portion, other than the device forming region, of a low-resistance Si wafer 5 having a resistivity of 0.003 .OMEGA.cm, the SiO.sub.2 film 6 serving as an isolation region. Next, heat treatment is carried out in an oxygen atmosphere to form an SiO.sub.2 film 7 having a thickness of 40 .ANG. on which is in turn formed a tungsten film 8 having a thickness of 1000 .ANG. by CVD (chemical vapor deposition) method. Then, patterning is carried out by dry etching. Finally, the tungsten film 8 is used as a source and a drain.
Next, as shown in FIG. 13A, Si atoms are supplied at a deposition rate of 0.1 .ANG./sec to the surfaces of the SiO.sub.2 film 7 and tungsten film 8 by an electron beam deposition method while heating is effected at 125.degree. C. in an ultra high-vacuum tank. Thus, on the surfaces of the SiO.sub.2 film 7 and tungsten film 8 are formed semispherical amorphous Si fine particles 9 of 20 .ANG. in diameter and 10 .ANG. in height at intervals of 20 .ANG.. Then, the temperature is raised to 500.degree. C. and heat treatment is carried out for 1 hour. Thus, the semispherical amorphous Si fine particles 9 are crystallized into fine particles 3 (crystallized Si fine particles).
Next, as shown in FIG. 13B, a boron/phosphorus-loaded glass 10 is deposited by a CVD method using SiH.sub.4, O.sub.2, PH.sub.3, B.sub.2 H.sub.5 as material gas, and the surface of the deposit is rendered flat by reflow through heat treatment at 800.degree. C. so that the resulting layer is 40 .ANG. thick at the region in which fine particles 3 are present and 50 .ANG. thick at the region in which particles 3 are not present.
Next, as shown in FIG. 13C, a tungsten film 11 is deposited to be formed into a gate electrode configuration. Then, as shown in FIG. 13D, an SiO.sub.2 film 12 is deposited by CVD technique to form an interlayer insulating film. Further, as shown in FIGS. 13E and 13F, wiring and passivation film forming are carried out, and then a source terminal 1, a drain terminal 2, a gate terminal 4, and a substrate potential terminal 13 are formed.
Next, applicability of above described quantum dots to light-emitting devices will be explained. FIG. 14F shows a section of a formed Si light-emitting device. The Si light-emitting device emits light when a voltage is applied between an upper electrode 15 and a lower electrode 16 to allow a tunnel current flow and carrier injection into crystal Si fine particles (hereinafter referred to as fine particles) 17.
An Si light-emitting device of above described configuration is formed in the following way. As shown in FIG. 14A, an SiO.sub.2 film 19 having a thickness of 2500 .ANG. is formed by conventional selective oxidization method on a surface portion, other than the device forming region, of a low-resistance Si wafer 5 having a resistivity of 0.003 .OMEGA.cm, the SiO.sub.2 film 6 serving as an isolation region. Next, as shown in FIG. 14B, heat treatment is carried out in an oxygen atmosphere to form an SiO.sub.2 film 20 having a thickness of 30 .ANG..
Next, as shown in FIG. 14C, Si atoms are supplied at a deposition rate of 0.1 .ANG./sec to the surfaces of the SiO.sub.2 films 19 and 20 by an electron beam deposition method while heating is effected at 125.degree. C. in an ultra high-vacuum tank. Thus, on the surfaces of the SiO.sub.2 films 19 and 20 are formed semispherical amorphous Si fine particles 21 of 20 .ANG. in diameter and 10 .ANG. in height at intervals of 20 .ANG.. Then, the temperature is raised to 500.degree. C. and heat treatment is carried out for 1 hour. Thus, the semispherical amorphous Si fine particles 21 are crystallized into fine particles 17.
Next, as shown in FIG. 14D, a boron/phosphorus-loaded glass 22 is deposited by a CVD method using SiH.sub.4, O.sub.2, PH.sub.3, B.sub.2 H.sub.5 as material gas, and the surface of the deposit is rendered flat by reflow through heat treatment at 800.degree. C. so that the resulting layer is 30 .ANG. thick at the region in which fine particles 17 are present and 40 .ANG. thick at the region in which particles 17 are not present.
Next, as shown in FIG. 14E, deposition of ITO (indium tin oxide) film 23 is carried out by a sputtering process and the ITO film 23 is etched by using a mask of a desired configuration as such to form an upper electrode 15. Further, as shown in FIG. 14E, the boron/phosphorus-loaded glass 22 is etched by using the upper electrode 15 as a mask, so that the boron/phosphorus glass 22 and fine particles 17 in portions other than the electrode region are removed. Further, as shown in FIG. 14F, wiring and passivation film forming are carried out. An Si light-emitting device is thus formed.
Further, a method of forming quantum dots of metallic material is described in a lecture document, entitled "Generation of a Aluminum Cluster Coated with .gamma.-Alumina" by Goto et al, which is contained in a collection of drafts for lectures made at "Ohyo Butsuri Gakkai" (the Japanese Applied Physics society) meeting for spring 1997, Lecture No. 28a-T-3, Lecture Collection p-1313. According to this method, a spherical aluminum cluster having a diametrical size of from 5 nm to 500 nm is formed by a magnetron sputter cohesion method such that aluminum (Al) is sputtered by DC discharge (220 V, 0.4 A) in argon (Ar) gas (4.times.10.sup.-3 Torr) so that Al cohesion can occur under the pressure of helium (He) gas (10 Torr) filled around the Al.
Next, one example of Si quantum fine wire forming method is explained. The method is described in a lecture document, entitled "Uniform Si Quantum Wires Fabricated by Anisotropic Etching on SIMOX Substrates" by Ishikuro et al, which is contained in a collection of drafts for lectures made at "Ohyo Butsuri Gakkai" (the Japanese Applied Physics society) meeting for spring 1996, Lecture No. 28a-PB-5, Lecture Collection p-798.
In the above noted Si quantum fine wire forming method,
i) as shown in FIG. 15A, after Si.sub.3 N.sub.4 film 26 is deposited on (100) SIMOX (separation by implanted oxygen) substrate 25, patterning is carried out in (110) direction; PA1 ii) as shown in FIG. 15B, by using the Si.sub.3 N.sub.4 film 26 as a mask, anisotropic etching is carried out with tetramethyl ammonium-hydroxide (TMAH) to form a (111) plane 27 on the pattern edge; PA1 iii) as shown in FIG. 15C, the (111) plane 27 is selectively oxidized by using the Si.sub.3 N.sub.4 film 26 as a mask to form an oxide film 28; and PA1 iv) as shown in FIG. 15D, after the Si.sub.3 N.sub.4 film 26 is removed, anisotropic etching is carried out with TMAH by using the oxide film 28 as a mask, whereby an Si quantum fine wire 29 extending in the (110) direction is formed. PA1 preparing an insulative substrate formed with a step, and selectively forming the ultrafine particles and/or ultrafine wire along the upper edge of the step. PA1 Ultrafine particles and/or ultrafine wire formed according to the above ultrafine particle and/or ultrafine wire forming method is used as the floating gate. PA1 an ultrafine semiconductor wire formed according to the above ultrafine particle and/or ultrafine wire forming method is used as the channel region.
As stated in a lecture document, entitled "Room Temperature Observation of Coulomb Blockade Oscillations in a Si Quantum Wire MOSFET Fabricated by Anisotropic Etching" by Ishikuro et al, which is contained in a collection of drafts for lectures made at "Ohyo Butsuri Gakkai" (the Japanese Applied Physics society) meeting for spring 1996, Lecture No. 26p-ZA-12, Lecture Collection p-64, observations on the dependence of drain current upon gate voltage in a device formed in manner as described above have witnessed a Coulomb blockade phenomenon due to one-electron phenomenon with respect to drain current as shown in FIG. 16.
Further, a method of forming a quantum fine wire of metallic material is described in a lecture document, entitled "AFM Fabrication of Al-Wire" by Sakurai et al, which is contained in a collection of drafts for lectures made at "Ohyo Butsuri Gakkai" (the Japanese Applied Physics society) meeting for spring 1997, Lecture No. 30a-PB-4, Lecture Collection p-515. According to this method, an Al film, 30 .mu.m wide and 8 nm thick, is vapor deposited on an SiO.sub.2 insulating substrate. Next, regions other than Al fine wire are oxidized by using atomic force microscopy (AFM). More specifically, Al portions other than Al fine wire are oxidized by applying a voltage between the AFM probe and the Al so that Al portions other than Al fine wire are oxidized to form an insulating film. Thus, an Al fine wire of 20 nm in width is formed.
However, in order that quantum dots or quantum fine wires formed by conventional quantum dot or quantum fine wire forming method, and an Si-based LSI may be loaded on a common substrate and allowed to integrally function, the following problems must be solved.
First, the quantum dot forming method disclosed in Japanese Patent Application Laid-open Publication No. 8-64525 is such that crystal particles of ultrafine particle size are formed on a substrate surface covered with an insulating film by utilizing an electron beam vapor deposition method, so that the condition of the substrate surface and the atmosphere (presence or absence of impurities) of the reaction chamber exert strong effect on the position and time for generation of a crystal nucleus which promotes growth of crystal particles, and particle growth after crystal nucleus generation. Therefore, it is very difficult to secure the position of crystal particle growth, particle size, uniformity of particle density, and reproducibility. As such, the method is unacceptable for mass production purposes.
The quantum dot forming method described in the lecture document "Generation of a Aluminum Cluster Coated with .gamma.-alumina" by Goto et al is a method which utilizes the process of sputtering and cohesion reaction in vapor phase. In the case of this method, too, it is very difficult to secure the position of crystal particle growth, particle size, uniformity of particle density, and reproducibility. As such, the method is unacceptable for mass production purposes.
The quantum fine wire forming method described in the lecture document "Uniform Si Quantum Wires Fabricated by Anisotropic Etching on SIMOX Substrates" by Ishikuro et al requires a very complicated process and involves various problems, such as high cost, low yield, and low productivity. Therefore, this method is unacceptable from the standpoint of realistic mass production.
The quantum fine wire forming method described in the lecture document "AFM Fabrication of Al-Wire" by Sakurai et al requires a very special type of miniaturizing technique which involves the use of an AFM (atomic force microscope), but in the current status of the art there exists no AFM which enables formation of fine wires at desired positions over the entire surface of the substrate. Another problem is how to form fine wires of uniform width with good reproducibility. Further, for development of necessary mass production equipment, there are many problems to be solved, including the problem of alignment and the securement of a realistic throughput.